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Extrapolation in calculated series

What is an extrapolation ? When to use it ?

Definition

Extrapolating means expanding the value of a tag after the last data point received for this tag.

Extrapolation and interpolations are two ways to “fill gaps” of data:

  • Interpolation = filling gaps between two points (i.e. two “real” points, actually received)

  • Extrapolation = filling in a gap after the last “real” point of a series, until a new point arrives

Applying extrapolation to calculated series can prevent seeing "no data" in your widgets.

Optional extrapolation feature

The purposes of this optional feature are to:

  • Extrapolation on:   
    Enable continuing a calculation with several tags even after one of the tags stops receiving data (it uses the last known value of the tag).
     

  • Extrapolation off:   
    Prevent the calculation as soon as any of the tag stops receiving data.

The only extrapolation we enable, today, is a “step”/”last value” extrapolation, i.e. the last known value of the tag is used as default to “extend” the tag to future dates for which no new point has arrived.

Let’s call:

  • “Last value” the value of the tag at the timestamp of the last point received for that tag.

  • “Last T” is the last timestamp for which a value was received

Calculated series with more than one tag

➡️ Let’s assume we want to create a calculated series = “A + B”: 

 

By default, when creating a new calculated series, the extrapolation option should be OFF.

When releasing the feature, previously created calculated series (all of them should be with extrapolation ON, since this was the default before the release) will continue to have extrapolation ON (so that users do not see changes in their calculations after the release). Newly created series will have extrapolation OFF by default.